meet only once; a once-met acquaintance ? 他的記憶力很好，我雖和他有～，但二十年后他仍記得我的名字。He has a long memory. Although I met him only once,he still remembered my name twenty years later.
半面之交bàn miàn zhī jiāo
指只見(jiàn)過(guò)一次面的人。only a chance acquaintance of somebody, a once-met acquaintance